Changing the state of a button on a slide

Hello :), I am trying to create a slide where my end users have 8 clickable pictures, at the beginning of the timeline the 'close' button is hidden, once they have clicked onto all the 8 pictures then the 'close' button is to become visible.  I can do this on the base layer but struggling to make it work on a slide... it might be that it cannot happen there? Thanks in advance Suey x